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Hey - you get replies to this thing? Never thought to look at it after I submitted it.
First - no, this one isn't available to lvl. 6 fighters. The BAB says so, true, but the skill requirements makes it possible only after reaching lvl. 8 - and in that case, your fighter has to have an INT of 16, in order to get enough skill points to get 2 cross-class skills brought up to 6 ranks (not a total skill-score of 6) - as well as having invested only in those skills over the course of his/her career. As skills don't grow from nothing (knowledge doesn't just pop up in the characters head), achieving this would actually reflect the character achieving an understanding of the undead state of being (knowledge: Undead) as well as the energies he/she comes to wield (knowledge: Arcana).
This should put the class-abilities in a different light concerning power-balance. Another limit on this class' powers is that in a setting with few undead, it becomes rather useless. If in doubt of the balance of the Spirit Stalker try comparing it to a paladin of the same level - it was aimed toward being the same.
As for why this class should be able to channel positive and negative energy: They have studied it. Instead of using their studies to achieve spells, they gain the ability to draw on what makes undead into undead - and what works against this state of being. They turn themselves into limited gateways to these two planes, in order to channel the energies. And no, other people than clerics can use positive energy - all cure particular wound spells are a sort of positive energy channeling.
You got a point on the negative energy combustion and the undead levels of experience.
I'm not that good at remembering where to find what in which book, but "no one is safe from turning into an undead creature"? Where's that written?
The last thing - the special requirement - was added to give this class a touch of vengeance-mongering. It is not a class a character seeks, as much as a class seeking the character. Examples fitting to this requirement could, besides survivors of lich-filled dungeons, be survivors of settlements ravaged by a powerful undead and its minions, those who have waged war against the undead... more or less anything, as long as powerful undead were involved.